Photoelectric structure is a structure whose electrical characteristics (e.g., current, voltage, or resistance) vary when electromagnetic radiation is incident upon it, as a result of a generation of electron-hole pairs in the structure caused by its interaction with said radiation. Such photoelectric structures are used in various semiconductor-based devices including photovoltaic type devices capable of converting solar or other thermal (infrared) energy directly into electricity, and photo-emissive type devices wherein charge particles (e.g. electrons, electron-hole pairs) are generated/emitted in response to input electromagnetic radiation.
The conventional approach to the configuration of photoelectric semiconductor-based devices consists of the use of P-N junction(s) for the generation of free charged carriers (e.g. electron-hole pairs) generated by the photoelectrical effect. The charged carriers are generated in all parts of the photoelectric structure: at the emitter (N-type part), at the base (P-type part) and at the so-called “depletion region” within the interface function) between the P- and N-doped layers in response to interaction with incident photons. However, only a little fraction of the electron-hole pairs generated at the emitter and the base regions can be collected. This is due to a high recombination rate in these regions. Thus, practically, the quantity of the charged carriers generated only in the depletion region defines the efficiency of the generation process and the possibility to collect these charged carriers. The generated charge carriers are forced to move through and away from the depletion region due to a built-in electric field existing in this region. More specifically, an electric field exists in the depletion region due to difference in the concentration of electrons and holes at both P and N parts of this region and due to the consequent diffusion of the charge carriers which tends to equilibrate these concentrations. This diffusion creates an internal electric field which eventually stops the diffusion and defines the depletion region dimensions and the internal electric field across this region. At this stage, newly generated charged carriers are separated and drifted in a suitable direction: electrons to the emitter (N) and holes to the base (P).
According to the conventional approach for building P-N junction based solar cells and also or P-i-N based photodiodes, the junction(s) containing structure is a stack of layers of different type conductivity stacked along an axis of light propagation through the photovoltaic cell, such that light passes across the junction(s). In other words, these existing structures are all the so-called “vertical” structures in which the generated charged carriers travel along the same direction as the photons being absorbed within the junction. Also, typically, Ohmic contacts are located on both sides of the cell to be above and below the junction(s), e.g. being in the form of a grid. Such Ohmic contacts, even if being in the form of a grid, reduce the effective surface area (exposed to interaction with photons) of the light collection surface.
Referring to FIG. 1, there is shown a cross sectional view of a bifacial photovoltaic cell structure (the figure is extracted from V. Everett et al., “Sliver Solar Cells”, http://solar.anu.edu.au/docs/Silver %20cells %20060621.ppt). The cell includes a slice of a conventional silicon p-type wafer. This p-type wafer has two Ohmic contacts associated with heavily doped regions, Boron-doped (p-type doping) on one side and Phosphor-doped (n-type doping) on all other surfaces of the slice. The active surfaces are the P-N junctions formed by interfaces between the p-type wafer and n-doped wrapping layer at other sides of the p-type wafer. When the cell is put in operation the P-N junctions thus serve as the active surface extending vertically along the optical path of light to which the cell is exposed.
Various semiconductor and compound semiconductor materials are quite useful in making photoelectric devices of the kind specified. Such materials, for example, are silicon, germanium, gallium arsenide, indium arsenide, indium antimonide, etc. Various semiconductor materials and/or different doping of the same semiconductor material provide higher sensitivity of a P-N junction in said material to a different spectrum of electromagnetic radiation. This property is used in various applications, for example it is known to build an array of photovoltaic cells with different spectral sensitivity (e.g. hetero-junctions).
Thus, the existing photovoltaic cells (or solar cells) as well as photodiodes, typically utilize a large-area P-N junction or P-i-N junctions, which is capable of generating usable electrical energy from light sources, including sunlight. These cells are typically—p-doped silicon wafer-based structures, for example prepared by the known technique utilizing sawing to wafers the Czochralski pulled ingots following the pulling process. Average impurity concentration (degree of doping) of these wafers (P-type silicon (boron doped) which is generally used for solar cells), serving as a baseline for the preparation of a P-N junction, is about 1016-1017 cm−3. Thus, the measured resistivity of a wafer is about 0.1-1 Ωcm.